Field of the Invention
The present invention is directed to a method to prepare a chloride-free magnesium electrolyte and a magnesium electrolyte containing the chloride-free active material.
Discussion of the Background
Magnesium batteries have been the subject of high interest and significant research and development effort in order to provide more economical, safer and higher capacity batteries to displace or supplement the conventional lithium batteries. Compared to lithium Mg potentially has a volumetric capacity of 3832 mAh cm−3 which is significantly greater than the 2062 mAh cm−3 of Li. Additionally, Mg has a negative reduction potential of −2.356V vs NHE. As the seventh most abundant element in the earth's crust, Mg has a lower resource cost and a lower environmental impact profile (see Aurbach: Nature, Vol 407, pp 724-727, 2000).
Significantly, Mg does not suffer from dendrite formation, which renders Li metal unsafe for commercialization as a high capacity anode material (West: Journal of Electrochemical Communications, Vol 155, pp A806-A811, 2008).
An ongoing objective in battery research is increasing the energy density beyond that offered by lithium ion batteries. This may require a shift towards batteries containing a pure metal anode. However, in the case of lithium, deposition occurs unevenly with formation of dendrites which leads to safety concerns during cycling. In contrast to lithium metal, magnesium metal deposition is not plagued by dendritic formation. Additionally, magnesium is more stable than lithium when exposed to air. However, magnesium has a reductive potential of −2.36 V vs. NHE and has a unique electrochemistry which precludes the use of magnesium electrolytes that are analogues of lithium electrolytes. Reduction of magnesium analogues such as Mg(PF6)2, Mg(ClO4)2 and Mg(TFSI)2 results in the formation of a blocking film on the magnesium anode surface through which successful deposition of magnesium has not been observed. (Feng, Z: Surface Coating Technologies, Vol 201, pp 3783-3787, 2006).
Reports of effective magnesium electrodeposition from Grignard reagents in ethereal solutions date as far back as 1927 and have periodically appeared in the literature ever since. In an attempt to enhance the stability of the electroplating baths based on Grignards, in 1957 Connor et al. investigated the electrodeposition of magnesium from magnesium borohydride Mg(BH4)2 generated in situ by the reaction of MgBr2 and LiBH4. Unfortunately, boron and magnesium co-deposit in a 1:9 ratio. Recently, Mohtadi et al. have demonstrated the use of magnesium borohydride as an electrolyte for magnesium battery. The oxidative stability of Mg(BH4)2 has been reported similar to Grignard solutions. However, one of the obstacles in developing high voltage rechargeable magnesium batteries is moving beyond the oxidative stability of Grignards such as ethylmagnesium bromide (EtMgBr) and butylmagnesium chloride (BuMgCl) which have an oxidative stability of 1.3V vs. Mg. The low oxidative stability of Grignard solutions limits the choice of available cathodes. In 1990, Gregory et al. synthesized an electrolyte Mg(B(C4H9)4)2 from the reaction of dibutylmagnesium and the Lewis acid tri-n-butylborane which showed enhanced oxidative stability versus BuMgBr. It was assumed that the character of the Lewis acid could be a factor in improving the voltage stability. Gregory also evaluated magnesium deposit quality by spiking of alkyl Grignards such as ethylmagnesium chloride (EtMgCl) and methylmagnesium chloride (MeMgCl) with aluminum trichloride (AlCl3) to enhance electrochemical plating.
Aurbach et al has popularized a novel class of electrolytes called magnesium organohaloaluminates. One such electrolyte called APC is generated in situ by the reaction of aluminum trichloride (AlCl3) with the Grignard phenylmagnesium chloride (PhMgCl) in a 1:2 ratio and has an oxidative stability in excess of 3.2 V vs. Mg and can deposit/dissolve magnesium with high coulombic efficiencies. All crystallized magnesium organohaloaluminates share the general cation(Mg2(μ-Cl)3.6THF)+and their redox stability is determined by their unique anions. Magnesium organohaloaluminate electrolytes possess a high oxidative stability on inert electrodes (above 3.0 V vs. Mg) such as Pt or glassy carbon and are capable of depositing and stripping magnesium at high currents. However, they have been reported to be corrosive towards less noble metals such as aluminum, nickel and stainless steel which limits charging in a coin cell battery configuration to under 2.2 V due to the utilization of such metals in the casing and current collector material. Since the oxidative stability of electrolytes governs the choice of cathodes it is of paramount importance to develop a non-corrosive magnesium electrolyte which will permit discovery of high voltage cathodes. Improving the voltage stability of magnesium electrolytes on stainless steel is crucial because stainless steel is a widely used current collector and a major component in a variety of batteries such as coin cells. Current state of the art magnesium organohaloaluminate electrolytes limit the usage of Mg battery coin cells to operating under 2.3V vs Mg. It has been well established that chloride ions are some of the most severe corroding ions which attack metal surfaces non-uniformly with the formation of pits. However, previous to this invention no report identifying the problem of chloride ion content of a magnesium electrolyte and describing effort to prepare a chloride-free magnesium electrolyte has been found.
Yamamoto et al. (U.S. 2013/0337328) describes a magnesium electrochemical cell containing a magnesium metal or metal alloy as a negative electrode, a graphite fluoride/copper positive electrode and an electrolyte which is a mixture of complexed magnesium metal ions in an ether solvent (1,2 dimethoxyethane), an alkyl trifluoromethanesulfonate, a quaternary ammonium salt and/or a methylimidazolium salt. The mixture also contains an aluminum halide (AlCl3) and after dissolving the magnesium and aluminum halide an ether complex of boron trifluoride is added. A range of boron salts are described as possible components. However, nowhere does this reference identify a problem due to chloride content of the electrolyte or disclose or suggest preparation of a chloride free magnesium complex salt as an electrolyte.
The present inventors (U.S. 2013/0034780) (U.S. Pat. No. 8,318,354) described the synthesis and structural identification of magnesium complex salts which may be the basic starting material for the present invention. However, conversion to a chloride free complex was not disclosed or suggested.
Singh et al. (U.S. 2013/0266851) describes a magnesium electrochemical cell having a negative electrode containing tin as an active component. Conventionally known electrolytes such as Grignard based systems and magnesium bis(trifluoromethanesulfonyl)imide are employed. However, nowhere does this reference identify a problem due to chloride content of the electrolyte or disclose or suggest preparation of a chloride free magnesium complex salt as an electrolyte.
Itaya et al. (U.S. 2004/0137324) describes an electrolyte for a magnesium battery that is composed of magnesium bistrifluoromethanesulfonimide in a nonaqueous organic solvent such as a carbonate, an ether or a molten salt. However, nowhere does this reference identify a problem due to chloride content of the electrolyte or disclose or suggest preparation of a chloride free magnesium complex salt as an electrolyte.
It was the inventors' initial hypothesis that one possible cause for the corrosive character of magnesium organohaloaluminate electrolytes is the presence of chlorides in the cation (Mg2(μ-Cl)3.6THF)+ of the magnesium salt. It was therefore an object of the present invention to discover a method to prepare chloride-free magnesium electrolytes which are compatible with non-noble metals such as Al, Cu and stainless steel for utility in a magnesium battery.
It was a further object to prepare chloride-free electrolytes for a magnesium battery.
It was a further object to provide magnesium electrochemical cells employing the chloride-free magnesium electrolyte and magnesium batteries containing the electrochemical cell.